For example, in a semiconductor device manufacturing process, there has often been used a plasma etching process, in which a predetermined layer formed on a target substrate such as a semiconductor wafer is etched by plasma, using a resist as a mask in order to form a predetermined pattern on the layer.
Although various kinds of plasma etching apparatuses have been used to perform such a plasma etching process, a plasma etching apparatus of a capacitively coupled parallel plate type has been mainly used.
The capacitively coupled parallel plate type plasma etching apparatus includes a chamber with a pair of parallel plate electrodes (upper and lower electrodes) provided therein. While a processing gas is introduced into the chamber, a high frequency power is applied to at least one of the electrodes to form a high frequency electric field between the electrodes. The processing gas is excited into plasma by the high frequency electric field, thereby performing a plasma etching process on a predetermined layer formed on a semiconductor wafer.
To be specific, there has been known a plasma etching apparatus for generating plasma in a desired state by applying a plasma generation high frequency power having a relatively high frequency and an ion implantation high frequency power having a relatively low frequency. With this apparatus, it is possible to perform an etching process with high selectivity and high reproducibility (for example, see Patent Document 1). In such an etching process, etching is performed by plasma, mainly including positive ions, of a processing gas.
Meanwhile, in order to meet a recent increasing demand for microprocessing, a film thickness of a photoresist used as a etching mask is getting thinner, and an ArF photoresist, which is exposed to a laser beam of which a light emitting source is an ArF gas having a shorter wavelength, has been widely employed. However, since the ArF photoresist has low plasma resistance, it has been difficult to form an etching hole with a sufficient etching selectivity.
To solve such a problem, Patent Document 2 discloses a technique of modifying an organic mask, such as an ArF photoresist, having a low etching resistance by implanting high-velocity electrons into a semiconductor wafer by means of applying a DC voltage to an upper electrode of a capacitively coupled parallel plate type plasma etching apparatus.
However, the effect of modifying the organic mask by the high-velocity electrons depends on a thickness of a plasma sheath on the semiconductor wafer. That is, a sufficient modification effect can be achieved in a process in which a high frequency bias power is low, so that the thickness of the plasma sheath is thin. However, when a high frequency bias power is high, the thickness of the plasma sheath would be increased so that the high-velocity electrons would be reflected from the plasma sheath. Accordingly, the high-velocity electrons may fail to reach the semiconductor wafer. As a result, a sufficient modification effect for the organic mask cannot be achieved by the high-velocity electrons.
Moreover, recently, along with the miniaturization of the semiconductor device, there has been a demand for HARC (High Aspect Ratio Contact) etching which features an aspect ratio equal to or higher than about 20. More recently, an ever higher aspect ratio beyond about 40 is demanded for next-generation HARC etching.
In such a HARC etching process, a photoresist as an etching mask is negatively charged. Thus, electric charges are neutralized on an etching surface in the early stage of the etching. As etching progresses, however, the aspect ratio increases, and, thus, the etching surface gets positively charged because positive ions are accumulated at the bottom of the hole. Therefore, positive ions, which play a key role in the etching processes, change its direction due to repulsion in the holes, resulting in warp or deformation of etching shapes. Further, as the bottom of the hole is positively charged, shading damage may be caused. Furthermore, since it becomes difficult for the positive ions to reach the bottom of the hole, an etching rate may be deteriorated.
Patent Document 1: Japanese Patent Laid-open Publication No. 2000-173993
Patent Document 2: Japanese Patent Laid-open Publication No. 2006-270019